The present invention relates to a display control device for storing contents such as characters, line drawings or photographs on books, newspapers or magazines in an electronic, optical or magnetic recording medium, reading them out from the recording medium and displaying them in a flat type display device.
Recently, portable terminal devices with a display control device for storing character data or graphic data including gray scale such as characters, line drawings or photographs on books, newspapers or magazines in an electronic, optical or magnetic recording medium, reading them out from the recording medium and displaying them in a flat type display device such as a liquid crystal display device or the like are researched and developed. These kinds of portable terminal devices comprise a display control device for storing data recorded in a recording medium in a semiconductor memory such as Video RAM, reading out the stored data and display the data with gray scale images in a flat type display device.
For such a display device, color display devices using a color CRT are popular. Recently, however, to satisfy needs for miniaturization of devices, laptop type or notebook type small devices are being produced. For display devices of the laptop type or notebook type small devices, liquid crystal display devices or plasma display devices not CRT are used, because the former have advantages on lightening and thinning devices.
By the way, liquid crystal display devices or plasma display devices are used as monochrome display device, because developed color display devices of these types are so far expensive. Details of an art for converting color display data to monochrome display data on the liquid crystal display devices or plasma display devices are disclosed on the Japanese Patent Laid Open No. 299020 (1990) and the Japanese Patent Laid Open No. 85687 (1991).
To display images in gray scale on a liquid crystal display device, recorded data are converted to weighted bit patterns and stored in a Video RAM. For the weighting of 8 gray scale display, for example, the display data of a picture element is patterned by 3 bit length data such as scale number 1 000!, scale number 2 001!, . . . , scale number 8 111!. When storing the scale data in the Video RAM, respective data areas such as second, first and 0-th bit planes of 3 bit length pattern are separately stored. A portable terminal device with a display control device for generating display data reads out the bit pattern stored in the Video RAM and generates scale display data.
The scale display data for a liquid crystal display is generated by decimating the data read out from the Video RAM. For the method of data decimation, it is possible to decimate frames in point of time from the read out data, then convert the result to display data. Displaying on the liquid crystal display device is performed by conducting a display picture element ON/OFF. The frame decimation is to control this ON/OFF operation in point of time.
FIG. 10 shows an example of 8 gray scale display of a picture element. In this case, 7 frames from frame number 1 to frame number 7 form a display cycle. For example, scale number 3 is displayed on a liquid crystal display device when the scale display data 0000011! is sent in point of time to the liquid crystal display device. Namely, frame number 1 to frame number 5 at the picture element position are OFF and frame number 6 and frame number 7 at the picture element position are ON.
In a display device using a liquid crystal display device, an interference between frequency of a fluorescent lamp and frequency of display data of the liquid crystal display device causes flickering. In addition, in a display control device controlling gray scale display of the liquid crystal display device, flickering of screen may occur due to the drop of frame frequency of display data or arrangement of display data. The reason is that eyes of user recognize ON/OFF of the display picture element.
For example, a case of outputting the same scale display data in an area is considered. In this case, if the above-mentioned scale number 3 display data is output, all picture elements in this area are simultaneously conducted ON/OFF in the same cycle. Eyes of user recognize this ON/OFF, thus flickering of screen is recognized. To decrease this flickering, the pattern of display data may be changed. For example, the pattern 0000011! of the above-mentioned display data is changed to 0010001! so as to increase frame frequency of display data. Then, the pattern of display data is changed for each line and each picture element. In the horizontal direction display data, the pattern of the display data is changed for each picture, for example, the odd number picture element on a display screen is changed to 0010001! and the even number picture element on a display screen is changed to 1000100!. In the same way as this, the pattern of vertical direction display data is also changed. In the vertical direction, the pattern of the display data is changed according to the line number.
FIG. 8 shows a construction of a conventional display control device 20'. As shown in FIG. 8, the conventional display control device 20' is provided between a Video RAM 22 for storing weighted bit data for gray scale display and a flat type display device 24 (a liquid crystal display device, in the present embodiment). It reads out data stored in the Video RAM, conducts decimation and outputs the result to the liquid crystal display device 24.
The display control device 20' comprises a first to third latch circuits 26, 27 and 28, a scale data generation section 30, an address generation section 32, a display control section 34, a control section 36', a scale table register section 38, decimation section 40' and a display data latch section 42. Display data that was output from the liquid crystal display device 24 is data decimated based a line number, a line discriminant signal and a picture element discriminant signal. The data to be decimated is stored in the scale table register section 38 of which construction is explained later. The data is read out from the Video RAM 22 and converted to scale data. This data selects a register in which an arbitrary value is written in the scale table register section 38. Based on this value, the data is decimated.
The Video RAM 22 is a semiconductor memory in which the data that has been weighted and separated for each bit plane is stored for each plane. In the present embodiment, three bit planes, that is, plane 3, plane 2 and plane 1 are provided. The first to third latch circuits 26, 27 and 28 respectively read out data of plane 3 to plane 1, and latch them. The scale data generation section 30 generates scale data for each plane from data latched in the latch circuits 26 to 28.
The address generation section 32 generates a read address of the Video RAM 22. The display control section 34 conducts reading from the Video RAM 22, control of the control section 36' and control of the liquid crystal display device 24. The control section 36' outputs control signals for decimation such as an odd/even number picture element discriminant signal, an odd/even line discriminant signal and a frame number to the decimation section 401. The scale table register section 38 connected with the CPU bus comprises four units of register of 8 bit length in which an arbitrary value can be written.
The decimation section 40' decimates register set values supplied from the scale table register 38, based on the frame number supplied from the control section 36', the odd/even number line discriminant signal, the odd/even number picture element discriminant signal and the scale data supplied from the scale data generation section 30, and outputs display data to the display data latch section 42. The display data latch section 42 latches display data output from the decimation section 40', and outputs the data to the liquid crystal display device 24 at a timing of a control signal that is output from the display control section 34. The liquid crystal display device 24 receives the display data and conducts ON/OFF of the display picture element of the liquid crystal panel at the timing of the control signal supplied from the display control signal 34.
FIG. 9 shows detailed constructions of the scale table register section 38 and the decimation section 40' that are shown in FIG. 8. Here, the example where 8 picture elements of the display screen are processed as a unit is explained.
The scale table register section 38 comprises a first to fourth scale table registers 51 to 54. The decimation section 40' comprises a first to eighth decimation units 61' to 68'. A signal writing data to the first to fourth scale registers 51 to 54 is supplied from a CPU (not shown).
The first to eighth decimation units 61' to 68' have the same construction, so only a detailed construction of the first decimation unit 61' is shown in this figure. The first decimation unit 61' comprises a first to fourth frame decimation sections 71 to 74, a first to fourth scale multiplexer sections 76 to 79, a first and second line decimation sections 81 and 82, and a picture element decimation section 84.
The first scale table register 51 is a register in which a scale table of display picture element of which line number is odd and picture element number is odd is stored. The second scale table register 52 is a register in which a scale table of display picture element of which line number is odd and picture elements number is even is stored. The third scale table register 53 is a register in which a scale table of display picture element of which line number is even and picture elements number is odd is stored. The fourth scale table register 54 is a register in which a scale table of display picture element of which line number is even and picture elements number is even is stored.
Based on a frame number as a selection signal, the first to fourth frame decimation sections 71 to 74 respectively select 8 scale table data of the same frame number from data of 8 scale 8 bit length (7 bits for frames and 1 bit for dummy) supplied from the first to fourth scale table registers 51 to 54. The selected scale table data is a data of 8 bit length (8 scale.times.1 frame). Each of the first to fourth frame decimation sections 71 to 74 comprises 8 multiplexers, construction of each of them is of 8:1.
Based on a scale data as a selection signal, the first to fourth multiplexer sections 76 to 79 respectively select a data of 1 scale and 1 frame from scale table data of 8 scale 1 bit length (1 bit frame) supplied from the first to fourth frame decimation sections 71 to 74. The selected data is a data of 1 bit length (1 scale.times.1 frame). Each of the first to fourth multiplexer sections 76 to 79 comprises a multiplexer of 8:1.
Based on an odd/even number line discriminant signal for discriminating an odd number line and even number line on the display screen as a selection signal, the first and second line decimation sections 81 and 82 select data supplied from the first to fourth scale multiplexer sections 76 to 79. The data supplied from the first and third scale multiplexer sections 76 and 78 are the data decimated in the first and third scale table registers 51 and 53 which are respectively displayed upon odd number line/odd number picture element on the display screen and even number line/odd number picture element on display screen. The data supplied from the second and fourth scale multiplexer sections 77 and 79 are the data decimated in the second and fourth scale table registers 52 and 54 which are respectively displayed upon odd number line/even number picture element on the display screen and even number line/even number picture element on the display screen. The second line decimation section 82 selects a data to be displayed when the number of the picture element of the display screen is even. The selected data is of 1 bit length. Each of the first and second line decimation sections 81 and 82 comprises a multiplexer of 2:1.
Based on an odd/even number picture element discriminant signal for discriminating an odd number picture element and an even number picture element on the display screen as a selection signal, the picture element decimation section 84 selects a 1 bit length display data from odd picture element display data supplied by the first line decimation section 81 and even number picture element display data supplied by the second line decimation section 82. The selected data is of 1 bit length, the picture element decimation section 84 consists of a multiplexer of 2:1.
Returning to FIG. 8, the data read out from the Video RAM 22 is latched for each plane in the latch circuits 26, 27 and 28. The data of which each plane data is latched in the first to third latch circuits 26, 27 and 28 are converted to a scale data of which most significant data is plane 3 data and least significant data is plane 1 data. The converted scale data is output to the decimation section 40'. Here, each plane data is of 8 bit length because 8 picture elements of the display screen is a processing unit. The scale data generation section 30 outputs 8 kinds of scale data of 3 bit length respectively, that is, totally 24 bits data.
As FIG. 9 shows, each of the first to eighth decimation units 61' to 68' is supplied by a 3 bit length scale data, a control signal output from the display control section 34 and totally 5 bit data, that is, a 1 bit length odd/even number picture element discriminant signal for discriminating odd number picture element and even number picture element of the display screen, a 1 bit length odd/even number line discriminant signal for discriminating odd number line and even number line of the display screen and 3 bit length frame number. Each of the first to eighth decimation units 61' to 68' conducts decimation to output 1 bit length display data, as mentioned above. The total 8 bits display data is latched in the display data latch section 42 and output to the liquid crystal display device 24 at a display timing controlled by the display control section 34.
As shown in FIG. 11, each data of the first to fourth scale table registers 51 to 54 is of 8 scale 8 bit length. In F10 to F87, display data are stored. The first to fourth frame decimation sections 71 to 74 respectively frame decimates data of the first to fourth scale table registers 51 to 54. The first to fourth scale multiplexer sections 76 to 79 respectively select data from data supplied by the first to fourth frame decimation sections 71 to 74, using scale data. The results are output to the first and second line decimation sections 81 and 82.
FIG. 12 shows an example of frame decimation when the frame number is 1 and data construction selected from the scale table register. The data selected in the first to fourth scale multiplexer sections 76 to 79 is a data stored in F31, for example.
When displaying images in gray scale data on the screen of the liquid crystal display device 24, using set values of the scale table registers 51 to 54 that are rewritten by a software, the above-mentioned conventional display control device 20' controls ON/OFF of the picture elements of the liquid crystal display device 24. In addition, to reduce flickering, it recognizes display positions and frame numbers of odd number picture elements, even number picture elements, odd number lines and even number lines, and controls ON/OFF of the picture elements of the liquid crystal display device 24.
In such a conventional display device, there are some problems such as necessity of set data in the scale table registers 51 to 54 even if there is no need to control ON/OFF of the picture elements of the liquid crystal display device 24 and increasing of circuit scale of the scale table register section 38 because of the register construction.